tweak mgcp_parse_audio_ptime_rtpmap()

- move the error logging up to the actual errors. Each appear only once, no
  goto labels needed.

- instead of strstr("rtpmap"), use osmo_str_startswith("a=rtpmap:") to more
  concisely trigger on the actual syntax of the audio parameters. Same for
  "a=ptime:".

Change-Id: I730111e245da8485c1b5e8811f75d140e379cec6
1 file changed